Field of the Invention
The present invention relates to a sewing machine that is capable of smoothly hemming a fabric retained in a tubular shape.
Background Art
Hitherto, as described in JP H10-235054 A, there is a sewing machine that is provided with a fabric folding device located on the front side of a needle hole. The fabric folding device includes a plate-shaped guide member that is movable in the right-left direction, a ruler member that is located on the right side of the guide member and has a curved surface corresponding to the guide member, and an air nozzle that urges a fabric by air pressure. The fabric can be folded in a gap between a right end part of the guide member and the curved surface of the ruler member by air streams ejected from the air nozzle in the state where the guide member and the ruler member are positioned close to each other with a certain distance therebetween. The folded fabric is fed to the needle hole and sewed.
When a tubular fabric (e.g., a hem or sleeve of a T-shirt, or a fabric for use in a torso portion of an undershirt) is to be hemmed using a sewing machine provided with the fabric folding device, a start point of a seam moves around a machine bed and comes into abutment with the fabric folding device as the hemming proceeds. In this regard, it is necessary to retract the guide member from the path of the start point of the seam before it comes into abutment with the fabric folding device. Otherwise, the start point of the seam may be caught or hooked by the front-side end edge of the guide member, as a result of which the fabric cannot be fed to the needle hole, and the hemming cannot proceed around the tubular shape. Accordingly, the sewing is failed and hence a sewed product becomes a rejected product.
In order to retract the guide member for avoiding such a sewing failure, a start point of the seam is optically detected in a conventional sewing machine. Although JP H10-235054 A does not specifically describe this detection, JP UM S 59-25349 B describes an optical detection in a sewing machine. Thus, the description is given with reference to JP UM S 59-25349 B. The sewing machine described in JP UM S 59-25349 B includes a combination of a phototransmitter and a photoreceiver, which is configured so that the photoreceiver receives light transmitting through the fabric, and detects that the fabric is correctly laid on each other, according to the brightness of the transmitting light received by the photoreceiver.
In a sewing machine provide with a fabric folding device like the sewing machine of JP H10-235054 A, the detection of the start point of a hemming seam is performed substantially in the same manner as above on the basis of the change in light received by the photoreceiver, which depends on whether the fabric blocks light or not. More specifically, a non-sewn portion of a fabric comes or sags down to a position between the phototransmitter and the photoreceiver and therefore blocks light, while a sewn portion of the fabric does not come or sag down to the position between the phototransmitter and the photoreceiver and therefore does not block light. Thus, the photoreceiver which starts receiving light can indirectly detect that a start point of the seam has reached a position between the phototransmitter and the photoreceiver through the detection of such sagging of the fabric, since there is a correlative relationship between the timing at which the start point of the seam has come and the timing at which the photoreceiver has started receiving light, and therefore it is assumed the start point of the seam has come.
A driving mechanism for driving the guide member is actuated upon detection of the start point of the seam to move the guide member away from the path of the start point of the seam. At the same time of detecting the start point of the seam, a counter mounted inside or outside the sewing machine starts counting the number of reciprocal motions (more specifically, the number of rotation of a main shaft of the sewing machine) and when the counted reciprocating motions reaches a certain number, a sewing operation is stopped.
However, in optical detection of the start point of the seam, the guide member can be artificially or manually retracted by, for example, an operator who lifts up the fabric located on the front side of the fabric folding device to separate the fabric from a position between the phototransmitter and the photoreceiver before the guide member is automatically retracted. Especially when the operator is skillful, he or she does such a thing in order to retract the guide member at an interval suitable in view of ease of operation. As a result, the timing at which the guide member is retracted may be varied depending on each operator, which leads to a difference in finished state of hemming among the operators. This is not preferable in view of constant sewing quality.